A laser robot, more specifically, a well known six-axis multi-articulated laser robot having six degrees of freedom of motion (J1 to J6) is provided with a robot wrist, i.e., a movable element attached to the extremity of the robot unit of the laser robot, and is additionally provided with an additional-axis mechanism having two drive motors to increase the degree of freedom of motion. The multi-articulated laser robot moves a machining head which is capable of emitting a laser beam for laser beam machining, using the additional-axis mechanism, along a fixed locus in a plane (two-axis) coordinate system.
Because most of the loci of laser-beam machining are circular and each has a comparatively small diameter, the applicant of the present patent application previously proposed a laser robot capable of forming small holes in the workpiece by precision laser beam machining, and the proposed laser robot is in the initial stage of practical application.
The robot unit of the six-axis multi-articulated laser robot provided with an additional-axis mechanism has the general construction shown in FIG. 2, and, as is generally known, the actions of the robot unit of the robot are controlled during laser beam machining operation by a robot controller 10.
The robot unit 1 of this laser robot has a robot body 3 set in an upright position on a robot base 2, a rotatable robot body 4 supported for rotation (J1) in a horizontal plane on top of the robot body 3, a robot upper arm 5 having one end pivotally joined to one end of the rotatable robot body 4 for turning (J2) about a horizontal axis, and a robot forearm 6 pivotally joined to the extremity of the robot upper arm 5 for turning (J3) about a horizontal axis relative to the robot upper arm 5.
Attached to the extremity of the robot forearm 6 is a three-degrees-of-freedom robot wrist 7 capable of turning (J4 to J6) in a three-dimensional space about three orthogonal axes, and an additional-axis mechanism 8 attached to the robot wrist 7 is provided with a machining head 9 that projects a laser beam.
The additional-axis mechanism 8 is provided with two built-in servomotors, i.e., drive motors, not shown, and controls the laser beam projecting nozzle 9a of the machining head 9, for example, for movement along a desired locus in the plane of an orthogonal two-axis coordinate system according to commands provided by a robot controller 10 to carry out a laser beam machining using a laser beam, such as a cutting operation or a boring operation.
This additional-axis mechanism 8 is principally used for forming holes having small diameters in a workpiece using the machining head 9. The additional-axis mechanism 8 holds the laser beam projecting nozzle 9a of the machining head 9 stationary at a predetermined original position with respect to the additional-axis mechanism 8 while the movable elements of the six-axis system (J1 to J6), i.e., the robot rotatable body 4, the robot upper arm 5, the robot forearm 6 and the robot wrist 7, of the robot unit 1 are in operation.
At the beginning of a laser beam machining operation, the robot unit 1 operates according to a machining program stored beforehand in the robot controller 10 by teaching to bring the laser beam projecting nozzle 9a of the machining head 9 into a position corresponding to the center of a small hole to be formed in the workpiece and to focus the laser beam on the center of the small hole to be formed. Then, the two built-in drive motors are actuated to move the laser beam projecting nozzle 9a and to position the same at a starting point on a desired machining locus for forming the small hole, and the machining head 9 is then moved along the machining locus for a laser beam boring operation.
Nevertheless, the thickness of the workpiece is not necessarily always the same since different parts of the workpiece have different dimensions and different portions of a piece of workpiece have different thicknesses, and the workpiece is not necessarily located exactly at a fixed position in the machining station. Accordingly, the laser beam is defocused regardless of the correct laser beam machining operation of the machining head 9 according to the predetermined machining program, and stable laser beam machining cannot be attained. Consequently, it often occurs that different processed pieces of workpiece are finished to different machining accuracies and have different qualities. It has been a conventional procedure for preventing such difference in machining accuracy and quality between pieces of workpiece to measure the longitudinal distance (vertical distance) between the laser beam projecting nozzle 9a of the machining head 9 and the surface of the workpiece after the machining head 9 has been located at a position corresponding to the center of a desired machining locus by the robot actions of the robot unit 1, so as to correct the longitudinal position of the laser beam projecting nozzle 9a of the machining head 9 on the basis of the difference between a longitudinal distance included in the machining program taught in advance to the robot controller and the measured longitudinal distance, and to refocus the laser beam.
However, the conventional method of measuring the longitudinal distance has the problems as set forth below.
(1) When a method of measuring the height of the laser beam projecting nozzle of the machining head of a laser robot from the work surface of a workpiece with a capacitance type height sensing device attached to the extremity of the machining head is employed, particles sputtered from the workpiece and smoke produced by machining during laser beam machining adhere to and deposit on the extremity of the capacitance type height sensing device to change the capacitance of the capacitance type height sensing device gradually and, consequently, errors are introduced into the measured distance and hence inaccurate correction results. PA1 (2) Another method of measuring the longitudinal distance employs an optical height sensing device, which is attached to the machining head with its optical axis inclined at an angle to the center axis of the machining head so that the optical axis of the optical height sensing device meets the center axis of the machining head at a working point on the surface of the workpiece. This height sensing device must unavoidably be attached to the machining head so as to jut out from the side of the machining head, causing mechanical interference between the height sensing device and various articles and elements surrounding the machining head. Furthermore, since the optical height sensing device projects a measuring beam obliquely onto the surface of a workpiece, the optical height sensing device is able to receive only part of the projected measuring beam and, consequently, a measurement error is introduced into the measurement. PA1 a bracket unit for mounting the height-sensing unit in close contact with the machining head so as to be in parallel with a center axis of the machining head; PA1 a measuring head unit incorporated in the height-sensing unit for automatically measuring a longitudinal distance between the height-sensing unit and a surface of the workpiece; PA1 a unit for writing known data representing a distance between respective center axes of the measuring head unit and the machining head, and a longitudinal distance between respective tips of the measuring head unit and the machining head in a storage unit provided for a robot controller; and PA1 a signal transmitting unit for transmitting, via a feedback signal line, data representing the longitudinal distance measured by the measuring head unit of the height-sensing unit to the robot controller.